This invention relates to the general field of gas turbines and more particularly to a gas turbine operating in more than one pressure mode.
A gas turbine would include a turbine, a compressor, and a combustor, plus a recuperator (heat exchanger) where higher efficiencies are desired in low pressure ratio gas turbines. Basically, air is compressed in the compressor, heated in the recuperator, mixed with fuel and burned in the combustor, and then expanded in the turbine. The turbine drives the compressor and the turbine exhaust or discharge provides the heat for the recuperator to heat the compressed air from the compressor.
With high power-to-weight ratios, high reliability, and low maintenance, gas turbines dominate commercial and military aircraft propulsion. They also dominate aircraft auxiliary power units and large military tank propulsion. But when it comes to commercial and personal land vehicles, such as buses, trucks and passenger cars, gas turbine applications have been extremely limited.
While virtually every automobile and small gas turbine manufacturer in the United States, Europe and Japan has built and tested gas turbine propulsion prototypes, none have seen production in commercially viable quantities. The problems have always been initial cost, fuel consumption, and response time.
When a gas turbine is used with an electrical generator, the combination is generally referred to as a turbogenerator; with the smaller versions called a microturbine. In a microturbine, the generator would normally be a permanent magnet rotor rotatably driven by the turbine within an electrical winding stator.
Microturbines are being successfully used in production commercial land vehicles. The Capstone Turbine Corporation of Chatsworth, Calif. now produces a microturbine, which is the primary source of power in hybrid electric buses and similar vehicles. These microturbines are economically sound while demonstrating long life with dramatically reduced maintenance and emissions. While production of these vehicles is still limited today, it is increasing and the microturbine is being proven in over-the-road revenue service.
However, to reach high production in a broad range of vehicles, including passenger cars, it is recognized that certain improvements will be needed. These include lower initial cost, significantly faster response time, and higher efficiency into the forty percent (40%) range at both full load and part load.
So why are microturbines successful in hybrid electric buses? First of all, cost is far less of an issue because electric buses are expensive and microturbines are a small percentage of the cost. In addition, electric buses have limited range, especially when air-conditioned. They must carry batteries that are typically one third of the weight of the bus. Microturbines provide essentially unlimited range, even when the air conditioning is on, and allow for a smaller, lighter, less expensive battery pack.
Next, the traditional problem of fuel consumption is greatly ameliorated because the microturbine can always operate at its most efficient point. Even when the bus is stopped, the output can be used to charge the batteries. In addition, the microturbine can be relatively small as it need only provide the average power. Most of the peak power required to accelerate the bus comes from the battery. The net result is that fuel consumption is typically one half and sometimes one third of that of conventional buses. Additionally, the problem of response time is eliminated because the battery provides the surge of power necessary to accommodate sudden loads.
It must also be recognized that the single most important attribute of the microturbine is its low emission levels. The California Air Resources Board (CARB) has approved fourteen manufacturers to sell heavy-duty diesel engines under 400 hp in California. NOx emissions of the best of these engines range between 3.2 and 3.8 g/bhp/hr. CARB has certified the diesel-fueled Capstone Turbine microturbine at less than one quarter of these levels or 0.7 g/bhp/hr using no catalysts or after treatment, where xe2x80x9cgxe2x80x9d represents grams, xe2x80x9cbhpxe2x80x9d is brake horsepower and xe2x80x9chrxe2x80x9d is hours.
Similarly, CARB has certified ten manufacturers to sell natural gas and LPG fueled engines. The NOx emissions of the best of these engines range between 1.3 and 2.4 g/bhp/hr or roughly double that of the Capstone Turbine microturbine operating on diesel. CARB has certified the Capstone microturbine at 0.53 g/bhp/hr on LPG and 0.26 g/bhp/hr on natural gas.
Microturbines have also been successfully applied in a wide range of applications other than hybrid vehicles with over one thousand seven hundred Capstone Turbine units delivered to date.
Capstone has shipped more than 1,700 microturbines to customers worldwide and these microturbines have logged more than 1,000,000 hours of commercial operation. Individual units have run for more than 20,000 hours in non-vehicular applications, such as distributed generation, with no maintenance other than fuel filter and air filter changes, with some of these runs at maximum output. Considering if these units were powering automobiles at only 80 kph (50 mph), this would be the equivalent of running 1,600,000 kilometers (1,000,000 miles) with essentially zero maintenance.
Existing microturbines drive permanent magnet generators that are integrated into the basic design and therefore have no mechanical drive capabilities. Fortunately, electric propulsion systems are now well developed and very efficient over a broad range of speeds. In any event, much and sometimes most of the energy consumed by a bus is for auxiliaries such as air conditioning, air compressors, lights and fans that can be driven electrically. And the thrust in all vehicles, including automobiles, is towards larger electric loads as all-electric power steering, power brakes and other auxiliaries are developed.
Hybrid electric buses are an excellent application for existing microturbines and batteries. However, if production of microturbines is to reach automotive quantities, the bulk, weight and cost of the battery pack must go. When the microturbine has to operate without an energy storage system (battery), higher efficiency at full load will be needed. But, even more important, with no battery, much higher efficiency at part load will be critical. In addition, the response time must be improved and the initial cost must drop.
For purposes of discussion, all calculations will be based on the following assumptions unless otherwise noted:
No allowance is made for change in pressure drop due to differences in recuperator effectiveness, as this is a function of recuperator design.
A microturbine using the above parameters would produce 74.3 kW at thirty-one and one half percent (31.5%) efficiency. While the microturbine is a very simple device conceptually, in practice, it has very sophisticated engineering. Thus, except for the fuel pump and possibly a cooling fan for the electronics, the only moving part is the rotor group, which includes the turbine wheel, compressor wheel and permanent magnet rotor. When the rotor group or spool is mounted on compliant foil fluid film bearings, there is no lubrication system and, indeed, no oil, no oil pump, no oil cooler and no need for oil servicing. As the microturbine is air cooled, there are no fluids in the machine other than fuel, and no turbine-driven accessories. This creates a compact package that operates over a limited speed range and is well suited to vehicles using electric propulsion.
In large gas turbines, the roads to high power and efficiency are: 1) increasing component efficiencies, 2) increasing pressure ratios, and 3) increasing turbine inlet temperatures. Unfortunately for small gas turbines such as microturbines, efficiencies of small components will never be as high as those of large components. Also compressors with small airflows cannot be designed with as high a pressure ratio as compressors with large airflows, and still be efficient. Finally, the turbine inlet temperature is limited by the use of the recuperator that small gas turbines must use if they wish to have competitive efficiencies.
Using a recuperator, a heat exchanger that transfers heat from the gas turbine""s exhaust to the compressor discharge air before this air goes into the combustor, microturbine efficiency can be improved by reducing the fuel required. The ratio of recuperator air inlet temperature minus compressor discharge temperature to turbine discharge temperature minus compressor discharge temperature is known as recuperator effectiveness. Eighty-five percent (85%) is a typical goal and in a typical small gas turbine, this will halve the fuel consumption or double the efficiency.
The disadvantages to using recuperators are: 1) they are heavy, often doubling the weight of the microturbine, 2) they are expensive, far and away the most expensive component in the microturbine and 3) they are limited in the temperature that they can take. As the recuperator inlet temperature is the same as the turbine discharge temperature, the turbine discharge temperature must also be limited. As the turbine inlet temperature is related to the turbine discharge temperature by the pressure ratio and turbine efficiency, the turbine inlet temperature must also be limited. This effect is even more pronounced at part loads where the rpm is reduced thus lowering the pressure ratio. Limiting the turbine inlet temperature limits the power and efficiency of the microturbine.
The present invention is directed to a gas turbine that operates in more than one pressure mode. During various system operating requirements, the gas turbine may operate in a positive pressure mode, a transatmospheric pressure mode, or a subatmospheric pressure mode. Valving is provided to control the particular pressure mode of operation in response to system requirements and to switch between pressure modes as required. The gas turbine may include a single fixed spool, multiple fixed spools, or a combination of fixed spool(s) and a free turbine. In addition, the gas turbine can be started in a mode at or below atmospheric pressure mode and can utilize a catalytic combustor.